1. Field of the Invention
The present invention relates generally to a relay control circuit, and in particular to a relay control circuit capable to suppress sparking caused by instant engagement between contacts of the relay.
2. Description of the Prior Art
Electromagnetic relays have been widely used in controlling electrical appliances. A relay comprises a casing in which a coil is mounted. The coil can be activated/deactivated to close/open mechanical contacts of the relay for supplying/cutting off power to the electrical appliances.
Although newly-developed devices, such as SCR and TRIAC have replaced the relays in some of the applications thereof, the relays are still prevailing in certain applications due to theirs low cost and easy operation.
A disadvantage that the relays commonly suffer is the sparking caused at the instant when the contacts of the relays get into engagement with each other or when the engagement is broken. The sparking causes noises and reduces the service life of the relays.
It is thus desired to provide a relay control circuit capable of overcoming the above problem.
An object of the present invention is thus to provide a relay control circuit that is capable to suppress sparking caused in the operation thereof.
Another object of the present invention is to provide a relay control circuit that is capable to compensate/correct performance problem caused by aging/deterioration in suppressing sparking.
To achieve the above objects, in accordance with the present invention, there is provided a relay control circuit connected between a power source supplying an alternate current having a voltage and a load to which the alternate current is supplied. The relay control circuit comprises a pair of contacts for conducting/cutting off the alternate current supplied from the power source to the load. The relay control circuit comprises a first zero-crossing detection circuit connected to the power source and generating a first zero-crossing signal to a control unit having a memory unit storing a time delay parameter. The control unit issues a control signal to a driving circuit for energizing a coil to actuate the contacts at a time point that leads the zero crossing point of the alternate current a time period equal to the time delay parameter whereby the contacts are actually actuated at a time point corresponding to the zero-crossing point. A second zero-crossing detection circuit is coupled to the load and generates a second zero-crossing signal. Theoretically, the second zero-crossing signal is received by the control unit at the instant corresponding to the time delay parameter after the control signal is issued by the control unit. In case of deterioration of the performance of the relay control circuit, a difference exists between the time of detection of the second zero-crossing signal and the time point of the time delay parameter after the control signal. The difference is used to correct the time delay parameter. The corrected parameter is stored in the memory unit for further use in actuating the contacts.